Chemical and metallurgical aspects of arsenical bronze: the case of arsenic-loss in prehistoric metal production

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Visit to Ross Island, Ireland

Following an invitation of the University of Cork, I was able to visit the Ross Island copper mines beginning of March.

The prehistoric copper mines, which date back as far as 2400 BC, are located in the Killarney National Park. The mines are situated at the shore of the lake on an peninsula, less than half an hour walk from Ross Castle, a 15th century fortress. One can arrive to the mines by following a trail, passing through swamp and a wild forest, and the area of recent mining activity from the 19th century.

It is still possitble to see lots of traces from prehistoric mining on site, such as working ‘tables’, grinding stones, and remnants of fire setting. Unfortunately, the full extend of the underground mining is not anymore accessible today due to floods and roof collapses. Ceramic found on site connect the mining site with the Beaker material culture. Close to the mines, just a few meters higher above the lake, the miner’s work camp, which was only seasonally used, was found. Here, the minerals were crushed, hand-sorted, and smelted in shallow pit furnaces. No slags were found. The ingots produced were then transported to the nearby settlements and cast into other objects.

The arsenic content of the main copper mineral mined, tennantite, reaches up to 20%. Of course most of it was reduced during smelting, but the final objects still show significant amounts of arsenic (1-5 wt.%) and a distinctive impurity pattern (As>Sb>Ag). This, and the high amounts of arsenic, can be traced in the so-called ‘A’-copper, which circulated widely in Ireland in the period 2400-1900 BC, and also reached Britain. Ross Island is so far the only Irish copper mine with prehistoric mining known with such high amounts of arsenic, and is consequently most likely the source of this ‘A’-copper, which makes it the oldest known mine in Ireland.

After my presentation, we visited Ross Island with Prof. William O’Brien. Wind, rain, and temperatures below -10°C (at least that what it felt like for me) did not stop us to collect some tennantite, which I intend to use for several smelting and melting experiments.

Ross Island, relicts of prehistoric copper mining activities.

…the minerals collected were good, as I realised the next day at the costums at Dublin airport: security stopped me to see which kind of strange metals I have in my bag. Should be at least 40% copper, they said. I am going to figure that out soon!

Marianne Mödlinger

Marianne Mödlinger is a prehistoric archaeologist. Her main research interests include Bronze Age studies, the manufacture and usage of Bronze Age arms and armour, archaeometallurgy of copper alloys, and experimental archaeology.